Optical integrated circuits constituted from silicon photonic wire waveguides make possible improvements in integration density and lower costs, and therefore show promise as a next-generation technology. A silicon photonic wire waveguide has a silicon core of sub-micron size, and a cladding which typically comprises silicon dioxide. Due to the high refractive index differential between the core and the cladding, guided waves are strongly confined within the core. Therefore, sufficiently low bending losses can be achieved even at small bending radii on the order of several microns, making possible very high density integration of optical device.
Fabrication techniques for silicon optical integrated circuits are compatible with silicon LSI fabrication techniques which are highly suited to mass production. Consequently, employing such LSI fabrication techniques is promising in terms of lowering the cost of silicon optical integrated circuits.
A problem encountered with sub-micron scale silicon photonic wire waveguides is that, due to the very great mismatch in mode dimension between the optical fiber and external optical elements such as the semiconductor laser or the like, optical wave coupling is extremely inefficient.
One method of solving this problem is to employ a spot size converter (SSC) having a silicon pointed structure (also called a “taper”) as shown in FIG. 7. In a silicon pointed structure SSC, between the silicon photonic wire waveguide and a second waveguide—the shape of the optical mode of which is to be efficiently matched with an external optical element—there is disposed a section in which the structure of the silicon optical waveguide has a silicon pointed structure of progressively smaller line width and covered by the core of the second waveguide, whereby the mode shape of the light confined within the silicon photonic wire waveguide is progressively expanded within the covering core of the second waveguide. Through this function, highly efficient optical wave coupling to external optical elements can be achieved. (See Patent Documents 1, 2 and Non-patent Document 1)
A prerequisite for obtaining highly efficient optical mode conversion in an SSC having a silicon pointed structure is that the optical mode be sufficiently expanded in shape outside of the silicon core. A very fine pointed structure is necessary for this purpose.
It has been indicated that, in a case in which the height of the silicon core is 220 nm, there is a need for a width of 100 nm or less for the TE mode; for the TM mode, a width of 50 nm or less at the distal end would be desirable. However, particularly with ordinary silicon LSI processes employing photolithography, fabrication of such an extremely fine pointed structure is extremely difficult due to the limits of resolution in the photolithography used in the process.
For this reason, methods employing electron beam exposure techniques have been proposed (see Patent Document 2, Non-patent Document 1)
However, due to their very low production throughput, electron beam exposure techniques are not promising as production techniques for practical purposes.
Moreover, while in principle {good results} would be achievable by employing the leading-edge immersion ArF excimer stepper technique, a problem is that the immersion ArF excimer stepper technique requires extremely high process costs.
With the foregoing in view, the inventors proposed a double patterning method for achieving a pointed structure of a silicon photonic wire waveguide, by a process comparable to one that relies upon an electron beam exposure technique or immersion ArF excimer stepper technique, even where a low-resolution exposure device such as an i-line stepper or the like is employed. (see Patent Document 3, Non-Patent Document 2)
Through the double patterning method, a silicon pointed structure having a distal end width of 50 nm can be formed, even when an i-line stepper with a resolution limit of about 200 nm is employed.
This double patterning method is described below, citing fabrication examples 1 to 3.